1. Field of the Invention
The present invention relates to a microstrip array antenna including a dielectric substrate, which is usable as a transmitting antenna or a receiving antenna of various radio wave sensors such as a vehicle-mounted radar.
2. Description of Related Art
A microstrip array antenna constituted of strip conductors formed on a dielectric substrate is becoming widely used as a transmitting/receiving antenna of various radio wave sensors including a vehicle mounted-radar such as an adaptive cruise control system for its advantages of slimness, low cost and high productivity.
Meanwhile, since a microstrip line has a large transmission loss at high frequency, there has been a problem that it is difficult to embody a microstrip array antenna having a high gain at high frequency. Accordingly, it is proposed to use a series-feed microstrip array antenna in spite of its design complexity instead of a parallel-feed microstrip array antenna widely used for its design simplicity. For example, refer to Japanese Patent Application Laid-open No. 2001-44752.
FIG. 20 shows an example of a series-feed microstrip array antenna 100 as proposed by this Patent Document. The microstrip array antenna 100 has a structure in which strip conductors are formed on a front surface of a dielectric substrate provided with a conductive ground plate at its back surface. In more detail, as shown in FIG. 20, a plurality of rectangular radiating antenna elements 101, 102, 103, 111, 112, . . . , are projectingly disposed at regular intervals on both sides of a straight feeding strip line 120.
Each of the radiating antenna elements 101, 102, 103, disposed on one side edge (on the upper side edge in FIG. 20) of the feeding strip line 120 are projectingly disposed at an inclination of approximately 45 degrees to the feeding strip line 120. Each of the radiating antenna elements 111, 112, . . . , disposed on the other side edge (on the lower side edge in FIG. 20) of the feeding strip line 120 are projectingly disposed at an inclination of approximately −135 degrees to the feeding strip line 120.
Input power fed to the feeding strip line 120 from an input end (leftward end in FIG. 20) thereof propagates to a terminal end (rightward end in FIG. 20), while sequentially coupling to the radiating antenna elements 101, 102, 103, 111, 112, . . . . Accordingly, the input power gradually decreases toward the terminal end.
To achieve desired directivity by use of such a series-feed microstrip array antenna, each of the radiating antenna elements has to be designed independently, because the series-feed microstrip array antenna is excited by traveling wave, and accordingly the coupling factor differs from one radiating antenna element to another. The coupling factors of the radiating antenna elements can be controlled by adjusting the element widths thereof.
For example, when all the radiating antenna elements are formed to have the same shape and size so that they have the same coupling factor, the power radiated from the antenna decreases toward the terminal end, because the input power inputted from the input end decreases toward the terminal end.
It is possible that all the radiating antenna elements have the same radiation factor if the radiating antenna element closer to the input end has a smaller element width to have a smaller radiation factor, and the radiating antenna element closer to the terminal end has a larger element width to have a larger radiation factor, as is the case with the microstrip array antenna 100 shown in FIG. 20.
As exemplified above, conventional series-feed microstrip array antennas are configured such that each of the radiating antenna elements has an adjusted element width to have a desired coupling factor.
However, since the adjustable range of the coupling factor of each radiating antenna element having such a configuration is relatively narrow, there has been a problem that desired antenna characteristics (desired directivity, for example) cannot be achieved in some cases.
In addition, when the element width is increased to achieve a large coupling factor, since a high frequency current flowing in each radiating antenna element along its lateral direction increases, a radio wave emitted in the direction crossing the direction in which a main polarized wave is emitted (the longitudinal direction of the radiating antenna elements) increases. This causes a problem that the radiation level of a polarized wave emitted in the crossing direction increases.
Furthermore, since each radiating antenna element is directly connected to the feeding strip line, it is difficult to achieve impedance matching for each radiating antenna element, and accordingly, it is difficult for each radiating antenna element to exhibit a desired reflection characteristic.